This paper presents and analyses a practical phenomenon that digital processing of analogous signal brings about shaking. The cause of shaking is that some high-freguences are left over signal behind the D/A transformation filter. The formulas of amplitude shaking and phase shaking are derived. The factors which have an effect on the amplitude shaking and phase shaking are analyzed. The method to reduce shaking is given in this paper. The conclusion is of significance for designing a system of digital processing.